The invention relates generally to the production of optical fibers and more specifically to a method of producing optical fibers having enhanced photosensitivity.
Optical fibers are used in a variety of applications, including communications systems, optical computers, laser tuning devices, and the like. Optical fibers, generally having a diameter in cross-section of about 125 xcexcm are composed of at least two portions, a central core and a cladding layer disposed about the core. The cladding layer has an index of refraction which is less than that of the core, with a typical index variation from the core to the cladding layer being in the range from about 0.003 to 0.03.
Typically, in the manufacture of optical fiber, a glass layer, e.g. a glass rod is formed and molten glass is formed by suspending the rod in a furnace. The optical fiber is drawn from the molten glass by a capstan, or other mechanism to form the fiber. Conventionally, the draw temperature is between 2025xc2x0 C. and 2045xc2x0 C.
It is well known to form gratings, such as Bragg gratings or long period gratings, in optical fibers to change the characteristics of the fiber for various applications such as wavelength division multiplexing (WDM), filtering, gain flattening, laser tuning and the like. Such gratings can be formed by creating portions of the fiber having a different index of refraction with respect to adjacent portions of the fiber. The changes in index of refraction can be accomplished by exposing the fiber to a focused source of radiation, such as ultraviolet (UV) light in the wavelength range of 193 nm to 248 nm, inclusive. There are several theories on how UV exposure relaxation effect caused by the breaking of bending defects. Another theory is that the UV radiation induces a rearrangement of the molecular structure leading to compaction of the glass matrix. In any event, it is well accepted that UV radiation can be used to alter the index of refraction in optical fibers and several methods for doing so, such as the xe2x80x9cholographic methodxe2x80x9d, are well known.
In some cases, enhanced photosensitivity of optical fibers has been linked to the presence of draw induces defects (DIDs) which are transformed during the drawing process into Ge related oxygen deficient centers (GODCs). It is known to foster such transformation by adjusting drawn tension of the optical fibers.
A first aspect of the invention is a method for manufacturing optical fiber with enhanced photosensitivity comprising the steps of forming a molten layer of glass and drawing a fiber from the molten layer of glass at a temperature of between about 1900xc2x0 C. and 1995xc2x0 C.
A second aspect of the invention is an improved method for manufacturing optical fiber with enhanced photosensitivity of the type wherein the fiber is drawn from a molten layer of glass at a predetermined temperature and a predetermined tension and at a predetermined rate. The improvement comprises the step of lowering the temperature that the fiber is drawn at between about 2% and 3% while increasing said preselected draw tension.
A third aspect of the invention is a method for manufacturing optical fiber with enhanced photosensitivity comprising the step of drawing the fiber from a molten layer of glass at a temperature of about 1985xc2x0 C. and a tension of about 200 gm.